Laser material processing has many advantages, including high productivity, a non-contact nature of processing, improved quality, and high precision and mobility of the laser beam delivery point. Lasers are presently used for cutting, drilling, welding, brazing, surface annealing, alloying, hardening, and other applications. An optical fiber is frequently used to deliver a high-power and/or high-intensity laser beam to a target.
A fiber-delivered laser material processing system typically includes the following components: a laser, for example a fiber laser; an optical coupler unit, also known as a switch box or shutter box; a process fiber (also termed “delivery fiber”), typically 10-50 m long, in a reinforced cable and pluggable on both ends; and a process head. The process head is an optical assembly that includes a receptacle for the process fiber, optics for projecting the laser power, and any required accessories for laser-based processing, such as assist-gas ports. The laser emission is transmitted into the optical coupler unit, either through free space or through a separate optical fiber. The optical coupler unit launches the emission into the process fiber, which transmits the laser light to the process head. The process head projects the laser light onto a workpiece to perform the required processing task.
Such systems are used with many different process types, including cutting, welding, heat treatment, etc., and with many different material types, thicknesses, and geometries. The desired focal spot size, divergence, and beam quality at the workpiece vary widely depending on the process type and associated process parameters. Because the process head is typically an imaging device, constructed to create the best possible spot using the fewest components, the spot near the workpiece is typically an image of the spot, or more specifically the beam waist, at the process fiber output, scaled by the magnification of the process head. The product of beam-waist radius and divergence (half-angle) is an invariant called the beam-parameter product (BPP), expressed in units of millimeter-milliradians (mm-mrad).
To create different spot sizes and/or different divergences of the laser beam, one usually swaps out the process fiber with another fiber of different core size, or swaps the process head with another head of a different magnification. The process of physical swapping of the delivery fiber and/or the process head is time-consuming and inconvenient, and can result in contamination or even damage of sensitive fiber tips.
Trumpf GmbH, of Ditzingen, Germany, recently reported specially constructed delivery fibers containing two different-sized waveguides, one central core surrounded by a cladding layer and then by an annular second core, giving the user the choice of a round beam or a larger doughnut-shaped beam (PCT Patent Application WO 2011/124671A1), thereby providing two discrete BPP values. The gross selection available by this technique is usable for switching between cutting and welding, for example, but for fine-tuning a process or switching among a variety of process types, it is much more useful to have a continuous BPP variability.
Ortiz in U.S. Pat. No. 5,245,682 discloses a beam quality control system, in which the divergence at the output end of a delivery fiber is controlled by switching the divergence of the laser beam focused into the delivery fiber. Referring to FIGS. 1A to 1C, a fiber-coupled laser system 100 of Ortiz includes a laser 101, three lenses 102A-102C having different focal lengths, and a delivery fiber 103 having input and output ends 104 and 105, respectively. The lenses 102A-102C are mounted on translation stages 110 allowing the lenses 102A-102C to be inserted into a beam path 106 of a laser beam 107 emitted by the laser 101, one at a time. For example, in FIG. 1A the first lens 102A, having the largest focal length of the three lenses 102A-102C, is inserted into the beam path 106, resulting in the smallest beam convergence angle α of a converging laser beam 108 focused onto the input end 104 of the delivery fiber 103. In FIGS. 1B and 1C, the convergence angle α of the converging laser beam 108 progressively increases with the decreasing focal length of the second and third lenses 102B, 102C, respectively.
Due to its relative stiffness and absence of sharp bends afforded by a stiff housing, the delivery fiber 103 preserves ray angles inside the delivery fiber 103, resulting in divergence angles β of a diverging laser beam 109 at the output end 105 being close to the corresponding convergence angles α of the converging laser beam 108. Therefore, when the lenses 102A to 102C are switched, the divergence angle β of a diverging laser beam 109 changes in a step-like fashion, which allows one to switch the BPP of the diverging laser beam 109 delivered to a workpiece, not shown. Detrimentally, the Ortiz system requires the lenses 102A-102C and/or the input end 104 of the fiber 103 to be re-positioned or aligned with a micrometer precision, which limits the practical applicability of such a system. Further, the laser beam 107 needs to be interrupted for the switching to take place, which may be inconvenient or even detrimental in some applications. Furthermore, since it is the optical power density or intensity of the focused laser beam that is primarily responsible for the laser processing action, it is the spot size, and not just the output beam divergence, that needs to be varied.
Imakado et al. in U.S. Pat. No. 7,151,788 disclose a laser processing device, in which the BPP of the laser beam delivered to a workpiece is also varied by varying the input divergence, resulting in varying output divergence at the output of the delivery fiber. A condensing lens re-focuses the laser beam exiting the delivery fiber into a focal spot on a target. A focal spot size achievable by the condensing lens depends on the divergence of the laser beam exiting the delivery fiber, which suggests that the lens must have significant levels of aberration. As a result, when the input divergence is varied, the focused spot size is varied. Detrimentally, when such a system is adjusted for high BPP, the focused spot will be blurry and will lack a well-defined edge that is desirable for machining. Additionally, the variation of the spot size as a function of divergence will likely be relatively modest; the divergence change will probably make much more of a contribution to the overall BPP change than will the spot size.
The prior art lacks a laser material processing system, in which the BPP, and in particular the spot size, would be varied smoothly, continuously, and in real time, with a well-defined focal spot. Accordingly, it is a goal of the invention to provide such a laser material processing system and method.